In optical communications networks, optical fibers are used to carry optical data signals between optical communications devices connected on opposite ends of optical fibers. In some optical communications networks, a device known as a 2-to-1 optical coupler is used to optically couple signals between an end of an optical fiber and transmit and receive channels of an optical communications device. The 2-to-1 optical coupler is generally an optical splitter comprising a branch-like structure having first, second and third ends. A first branch of the optical coupler extends between the first and second ends of the optical coupler. A second branch of the coupler extends between the first and third ends of the optical coupler.
2-to-1 optical couplers are used in a variety of applications, including, for example, bi-directional communications over POFs. When used for bi-directional communications, the first end of the optical coupler is connected to a first end of a main POF and the second and third ends of the coupler are connected to transmit and receive sides, respectively, of an optical transceiver. In a transmit mode, optical data signals generated on the transmit side of the optical transceiver are passively routed over the 2-to-1 optical coupler from the second end of the coupler to the first end of the coupler. As the optical data signals arrive at the first end of the optical coupler, they are optically coupled into the first end of the main POF. In a receive mode, optical data signals that pass out of the first end of the main POF pass into the first end of the optical coupler and are then passively routed along the second branch of the coupler from the first end of the coupler to the third end of the optical coupler. As the optical data signals pass out of the third end of the coupler, they are received in the receive side of the optical transceiver.
FIG. 1A illustrates a side view of a portion of a typical POF bi-directional optical communications link, which includes a 1.0 millimeter (mm) POF 2 and a 2-to-1 optical coupler 3. The POF 2 functions as the main optical fiber of the link. In one direction, the 2-to-1 optical coupler 3 routes optical signals generated by a transmitter (Tx) onto an end face 2a of the main POF. In the other direction, the 2-to-1 optical coupler 3 routes optical signals passing out of the end face 2a of the main POF 2 onto a photosensor (not shown) of receiver (Rx) 6. The Tx 5 and the Rx 6 are typically parts of an optical transceiver module (not shown). The main POF 2 is referred to as a 1.0 mm POF due to the fact that the diameter of the core of the POF 2 is 1.0 mm. The 2-to-1 optical coupler 3 has the branch-like splitter configuration described above, with each branch comprising a respective branch POF 3a and 3b. The branch POFs 3a and 3b are typically also 1.0 mm POFs. The end face 2a of the main POF 2 has a cross-sectional area equal to 8/32π, where π=3.14159. Likewise, the end faces 3c and 3d of the branch POFs 3a and 3b, respectively, have -sectional areas equal to 8/32π. However, the end faces of the branch POFs 3a and 3b that interface with the end face 2a of the main POF 2 are each reduced in cross-sectional area by approximately 50% to form a coupler end face 3e having a cross-sectional area of 8/32π, which matches the cross-sectional area of the end face 2a of the main POF 2.
FIGS. 1B and 1C illustrate front plan views of the end faces 2a and 3e of the main POF 2 and of the coupler 3, respectively. It can be seen from FIGS. 1B and 1C that the end faces 2a and 3e have equal cross-sectional areas. A variety of techniques may be used to reduce the cross-sectional areas of the end faces of the branch POFs 3a and 3b to form the coupler end face 3e. Polishing and chisel cutting are two well know techniques that are used for this purpose. In addition, in some cases a technique known as metal evaporation is used to form a metal layer 7 between the branch POFs 3a and 3b at the coupler end face 3e to prevent light from being coupled between the branch POFs, i.e., to prevent optical cross-talk. A configuration of the type shown in FIGS. 1A-1C is disclosed in U.S. Pat. No. 7,206,493. Another technique for varying the cross-sectional areas of the end faces of the branch POFs is a hot molding technique that uses a molding tool in combination with heat to provide the coupler end face with a desired non-circular cross-sectional shape. Such a technique is disclosed in U.S. Pat. No. 6,473,555.
One of the disadvantages of 2-to-1 optical coupler configurations of the type shown in FIGS. 1A-1C is that there is very little or no cross-sectional overlap between the end face 2a of the main POF and the end face 3e of the 2-to-1 optical coupler 3. The lack of overlap between the end faces 2a and 3e can lead to the occurrence of unacceptable optical insertion losses in the transmit and/or receive directions. Excessive insertion losses can degrade signal quality and can limit the length of the optical link. While the technique disclosed in U.S. Pat. No. 6,473,555 can be used to produce a 2-to-1 POF coupler having an end face with a cross-section that overlaps the cross section of the end of the main POF, due to the non-circular cross-sectional shape of the coupler end face, unacceptable insertion losses can still occur. For example, if the coupler end face has a cross-sectional area that is larger than that of the main POF end face such that there is cross-sectional overlap, the overlap may lead to improved optical coupling for light being coupled from the main POF into the coupler, but may lead to the occurrence of unacceptable insertion losses for light being coupled from the coupler into the main POF.
Accordingly, a need exists for a 2-to-1 POF optical coupler that provides a carefully-selected amount of overlap at the interface between the end face of the coupler and the end face of the main POF to reduce optical coupling losses in both direction in a bi-directional optical communications links.